In the art of preparing concrete, the water component plays a critical role, for example, the water-cement factor (W/C) determines the ultimate compressive strength of the concrete and other characteristics of the mix during and prior to setting. Apart from the need to establish a certain water-cement factor, commercial applications of concrete require that it be prepared with a predetermined consistency which is fully reproducible and maintained constant.
A difficulty, however, arises because the consistency of concrete mix cannot be established solely by the addition of a given amount of water thereto. The components of the mix, generally the sand and gravel, have varying moisture contents which may change from the start of the feed of the component to the mixer to the end or from charge to charge. The water supplied to the mix must be added in greater or lesser quantity, depending upon the moisture content of the other components, to obtain a predetermined consistency.
There are, of course, systems for controlling the consistency of a concrete mix. For example, in the German Pat. No. 17 84 920, electrodes are provided in the mixer to respond to the electrical resistance of the mix, the resistance being a function of the consistency.
In German Pat. No. 16 83 778, a system is described which responds to the power required to drive the mixer, this power demand being likewise a function of the consistency of the concrete mixture. Finally, in German patent document (open Application-Offenlegungsschrift) DE OS No. 27 12 210, a dielectric sensor for the moisture in the mix is provided to enable a control system to regulate the consistency.
Thus in modern concrete preparation, the electrode sensing of the electrical resistance of the mixture or the measurement of the mixer-drive power have found practical application and indeed systems for controlling the consistency of concrete mixtures heretofore have for the most part been based on one or the other of these measuring processes.
Practical experience with these techniques has, however, shown that there is a dependency of the electrical resistance of the mix or of the power demand for the mixer upon the consistency which is nonlinear. For example, the change in the output signal per increment of consistency change is initially relatively large and falls off with increasing water content of the mix as will be described in greater detail below.
In the case of electrical resistance measurements of consistency, the resistance falls off sharply with the beginning of water addition to a point at which further water addition does not materially change the measurement. In the case of power demand for the mixer, there is initially no reaction with the addition of water to the mix and only when a certain amount of water has been added is there a sharp drop in the power demand. At this point the power change per increment of consistency change is initially relatively greater and falls off with increasing water addition.
Furthermore, these experiments have shown that the magnitudes and patterns of the measured values depend upon the composition of the mix, the amount of the mix and other variables which detrimentally affect efforts to obtain a fully reproducible consistency of concrete mixes.
Neither of the two measuring techniques discussed above, moreover, appears to adequately encompass the total consistency range in concrete production. The values obtained are also subject to sharp fluctuations which can be superimposed on the measurements and tend to have a certain periodicity which depends upon the speed of rotation of the mixer. These fluctuations result, in turn, in high tolerances which must be observed to the detriment of accuracy in establishing a given consistency by conventional control techniques.
It has been proposed, for example, to provide RC (resistance/capacitance) networks to smooth out the superimposed fluctuations at least in part. Such networks have the disadvantage that they introduce a delay in the measuring process, the delay increasing the mixing duration with all of the disadvantages that can be expected from such an increase. For example, the increased mixing time results in greater energy consumption, more significant wear of the mixer and reduction in the mixing efficiency.
When the electrode system is used, moreover, a plurality of electrodes must usually be provided at respective locations in the mixing apparatus, e.g. the mixing drum or trough, and the output values or signals are averaged to provide an actual-value signal which is a mean of the signals from the various electrode stations. Naturally this need for a number of electrode stations and averaging increases the capital and maintenance cost since the electrodes are continuously subject to wear and must be frequently readjusted, repaired or replaced.
In conventional systems the control of the water addition generally is carried out in such a manner that water is continuously fed to the mix until the set point value of the consistency is obtained. Obviously this requires that the water be fed relatively slowly so that it can be readily blended with the remainder of the mix.
It has also been proposed to introduce continuously a portion of the necessary water over a first mixing period and then to interrupt the water feed for an additional mixing period, whereupon a reduced quantity of water is supplied, followed by another mixing time with the alternations of water feeding and interruptions continuing until the desired consistency is obtained. The mixing and water-feed intervals in many cases are held constant while in other cases they can be varied.
All the conventional control procedures outlined above have the common disadvantages that they require a relatively long time before the set point consistency or desired consistency is reached. Since the components of the mix are continuously agitated in the mixer during this period, the wear of the moving parts of the mixer and those parts which are contacted by the moving mixture is relatively high, energy consumption is also high and the mixing efficiency over the entire mixing period is low.
To improve the efficiency, larger mixers have been proposed although this has the disadvantage of higher capital cost and impracticability if smaller volumes of a mix are desired.